hoop conjecture

The hoop conjecture is a theory proposed by Kip Thorne in 1972 to predict whether or not any given object of any given shape will collapse to form a black hole. A tricky problem, as the collapse of highly non-spherical objects may, or may not form a black hole, depending on their exact geometry and mass.

It works like this: For the object in question, measure its mass, and then take a hoop with a circumference equal to the object's critical circumference. If the object can be placed in the hoop, and the hoop rotated around it so that the object and hoop never touch, then it must collapse to form a black hole.

This conjecture hasn't been finally proved yet, but every simulation of implosions of non-spherical objects has formed a black hole exactly when predicted, according to the conjecture.